The present invention relates to a two-stroke cycle engine, and, more particularly, to a two-stroke cycle gasoline engine adapted for use with automobiles.
A two-stroke cycle engine has theoretically the advantage that it can generate greater power for its size when compared with a four-stroke cycle engine, because the two-stroke cycle engine has twice as many work strokes per revolution as the four-stroke cycle engine. However, a conventional two-stroke cycle engine such as a two-stroke cycle gasoline engine which employs fuel-air mixture for scavenging has the drawback that it has high fuel consumption as compared with a four-stroke cycle engine, due to the loss of fuel-air mixture caused by the direct escape, i.e. blow-out, of scavenging mixture to the exhaust manifold during scavenging, in addition to the common drawback of two-stroke cycle engines that it cannot generate such a high power as expected from the fact that it has twice as many work strokes as the corresponding four-stroke cycle engine, due to the fact that the scavenging is still insufficient. Further, when the direct escape of scavenging mixture to the exhaust manifold occurs, HC content of exhaust gases increases, thereby making the problem of atmospheric contamination more serious.
In conventional two-stroke cycle gasoline engines, the rate of blow-out of scavenging mixture increases as the engine load or the delivery ratio increases, so that a large amount of HC is discharged in high load operation of the engine. On the other hand, in low load operation of the conventional two-stroke cycle gasoline engine, although the blow-out of scavenging mixture does not occur because delivery ratio is low and trapping efficiency is high, scavenging efficiency is low and a large amount of exhaust gases remain in the cylinder chamber, whereby fresh fuel-air mixture is much diluted by these remaining exhaust gases and is made less combustible, whereby combustion of this fuel-air mixture in the cylinder chambers becomes unstable, thereby causing torque fluctuation, power reduction, and increase of fuel consumption, so that the overall performance of the engine is much deteriorated. Further, in such operation of the engine a large amount of uncombusted fuel is discharged from the engine, so that the problem of HC contamination again becomes serious.
With respect to a two-stroke cycle gasoline engine it is known that, if scavenging fuel-air mixture is brought into contact with hot exhaust gases remaining in the cylinder during scavenging, a part of the fuel is decomposed by the heat of the exhaust gases so as to generate radicals such as C2, CH, OOH, COH, and H, which can self-ignite when compressed during the compression stroke of the engine. If this decomposition of fuel is available, stable combustion of fuel is ensured even in low load operation of the engine, whereby the aforementioned problems due to unstable combustion are completely obviated. In this connection, it is also known that, in fact, two-stroke cycle gasoline engines sometimes operate in low load high speed operation without depending upon ignition by an ignition plug. However, in most of these cases the real situation is not self-ignition by the generation of radicals, but is actually hot-spot ignition due to a hot spot generated in the wall of the combustion chamber due to some local overheating in the combustion chamber.